1. Field of the Invention
This invention relates to a foamed resin molded product, a foamed insulated wire, a cable and a method of manufacturing a foamed resin molded product.
2. Description of the Related Art
A wire using a fluorine resin for an insulator (so-called fluorine resin wire) has a high melting point and excellent solder dip resistance, thus it is used for a solder connection between a cable and a terminal and/or connector. In addition, the fluorine resin wire has excellent durability to environmental degradation such as chemical resistance, thus it is used for internal wiring of an electronic device such as a computer and wiring of a high-frequency device such as a mobile phone, a measurement device. Furthermore, the fluorine resin wire has excellent heat resistance and cold resistance, thus it is used for wiring of a high temperature device, and for a lead wire in a low temperature environment.
The conventional fluorine resin wire uses, as the insulator material thereof, polytetrafluoroethylene (PTFE), a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) or a tetrafluoroethylene-hexafluoropropylene copolymer (FEP). These fluorine resins are excellent in heat resistance, cold resistance, and chemical resistance, and also the resins have an extremely low relative dielectric constant as 2.0 to 2.1.
However, recently, in accordance with speeding up (transmission speed: not less than 10 Gbps/sec) of an electronic device and higher frequency (GHz band frequency) of a communication device, a need that the dielectric constant is further lowered occurs. Consequently, the lowering of dielectric constant is carried out by fibrillating a fluorine resin composition by means of foaming or stretching so as to make it porous.
Attempt to make it porous is mainly carried out in PTFE. For example, PTFE that has a tape-like shape, and a porous configuration by stretching is wound on the periphery of the inner conductor as an insulator, thereby the lowering of the dielectric constant is realized (refer to JP-B-H02-34735 (Utility Model Registration)). The PTFE tape having the porous configuration is mainly used for a small-diameter high speed transmission cable.
However, the adhesion property to the inner conductor is deteriorated, and so on, thus the characteristics are also deteriorated. In addition, the PTFE tape is wound such that many layers thereof are formed so as to increase a thickness of the insulation layer, accordingly, the production speed becomes slow and the production cost becomes high.
In addition, melt extrusion cannot be applied to PTFE, thus a method of manufacturing an insulated wire is adopted, the method having a configuration that PTFE powder is impregnated with a solvent such as solvent naphtha so as to become in paste form, and then the inner conductor is coated with the paste by using a paste extruder, after that, the solvent portion is vaporized and the PTFE is sintered in a baking furnace, thereby the insulated wire is obtained. The foamed insulator obtained by the paste extrusion is mainly used for a high frequency coaxial cable.
As a method using the paste extruder, there is a method, for example, the method having a configuration that a pore-forming agent such as dicarboxylic acid is kneaded together with the PTFE powder and the pore-forming agent is vaporized at the time of sintering so as to manufacture an foamed insulated wire (refer to JP-A-2011-76860).
However, in the foaming by using the pore-forming agent, the extent of foaming is low, and there is a problem that it cannot be used for a low-loss cable.
On the other hand, in case of PFA and FEP to which melt extrusion can be applied, a physical foaming method is used, the method having a configuration that in the middle of extrusion, an inert gas as a foaming agent such as “Freon gas” (registered trade mark), nitrogen gas, carbon dioxide gas is poured into a cylinder of the extruder, and difference in pressure at the time of discharging the materials is utilized, thereby the foaming is realized (refer to JP-B-4879613).
However, in case of using the physical foaming method, it is difficult to control the amount of gas used as a foaming agent, as a result, it is impossible to control the size of the bubbles. In a small-diameter foamed insulated wire, if the bubbles are excessively increased in size, the outer diameter variation thereof is also increased, thus the problem is caused that electrostatic capacitance and characteristic impedance are deteriorated. In addition, in a large-diameter coaxial cable, enormous bubbles (pores) are generated between the inner conductor and the foamed insulator along with an outer diameter abnormality, thus the problem is caused that Voltage Standing Wave Ratio (VSWR) of a benchmark in a longitudinal direction of the cable is deteriorated.
Furthermore, a chemical foaming method is also used, the method having a configuration that a chemical foaming agent that is foamed due to heating at the time of the melt extrusion is added to a resin compound, thereby the foamed resin is obtained. The chemical foaming agent is divided broadly into two categories of an inorganic chemical foaming agent and an organic chemical foaming agent. Major types of the inorganic chemical foaming agent include sodium bicarbonate and the like, and these compounds generate carbon dioxide that has high solubility in polymer at the time of decomposition thereof. However, as a decomposition product, a metallic salt that has high dielectric constant (∈) and high dielectric tangent (tan δ) is produced, thus it is difficult to use these compounds for a high speed transmission cable and a high frequency cable for which the lowering of the dielectric constant is needed. Consequently, an organic chemical foaming agent is mainly used.
The organic chemical foaming agent includes, for example, a bistetrazole-based compound such as bistetrazole diammonium, bistetrazole piperazine, bistetrazole diguanidine. A method of manufacturing a foamed insulator by using the organic chemical foaming agent includes a master batch (MB) method and a full compound (FC) method. The MB method is configured to prepare a foaming agent master batch (MB) in order to enhance dispersibility of the organic chemical foaming agent by adding the chemical foaming agent to the resin such that the concentration thereof is condensed to become approximately ten times of the amount used, and dilute this by adding a base resin to the amount used so as to form a resin foamed product. On the other hand, the FC method is configured to knead the chemical foaming agent and the total amount of the resin at a stroke so as to prepare a foamable compound, and feed this to a forming machine so as to form a resin foamed product.